The present invention relates to radio-frequency (RF) shields and, more particularly, to a novel double-sided RF shield for placement between an RF body coil and a set of gradient coils in a nuclear-magnetic resonance (NMR) imaging device.
An NMR imaging device typically utilizes a set of three gradient coils to obtain spatially-selective information. Each of the these gradient coils generally contain a multiplicity of turns of conductive wire, with total lengths of up to several hundred meters. RF fields lose a significant portion of their energy if these fields impinge upon the conductive wires of the gradient coils; while the loss mechanism is not fully understood, it is probably associated with high current resonances exciting the gradient structure and producing associated high losses. Any RF power loss, in the gradient coils or otherwise, appears as a lowering of the quality factor Q of the RF coil and consequently appears as a lowering of the signal-to-noise ratio (SNR) attainable in the imaging device. Accordingly, it is highly desirable to prevent penetration of the RF field into the gradient coils; a shield is typically placed between the RF coil and the gradient coils. The RF shield must, however, be substantially transparent to the gradient magnetic fields and therefore must prevent inducement of any significant shield currents at gradient frequencies (typically less than about 10 KHz.) to prevent temporally-dependent and/or spatially-dependent magnetic field inhomogeneities from appearing and having an adverse affect on the resulting image.
Hitherto, the most commercially used RF shield in a medical NMR imaging device has been a double-sided shield using a copper-dielectric-copper laminate sheet having an empirically obtained overlapping patchwork pattern etched into the copper sheets on both sides of the laminate. The patchwork pattern is generally a poor approximations to the current paths in a solid shield, so that induced currents are forced to flow through the shield dielectric at several locations. Thus, the current path contains the equivalent of several capacitors in series and total path capacitance is undesirable relatively small. It is desirable to not only provide a highly-effective, double-sided RF shield for placement between an RF coil and a set of gradients coils, but also to provide such an RF coil in which the shield current flows through as large an equivalent shield capacitor as possible.